A vehicle (so called an electric vehicle or a hybrid vehicle) that obtains a driving power from an electric motor includes a high voltage battery for driving the electric motor used as a power and a low voltage battery for operating auxiliary devices such as vehicle lightings or a radio. A DC-DC converter device that performs a power conversion from the high voltage battery to the low voltage battery or from the low voltage battery to the high voltage battery is mounted on the above-described vehicle. Hereinafter, the description will be made under the assumption of the power conversion from the high voltage battery to the low voltage battery. However, the direction of the power conversion is not limited thereto.
Generally, a DC-DC converter device has a circuit configuration to include a high voltage side switching circuit that converts a DC high voltage to an AC high voltage, a transformer that converts the AC high voltage to an AC low voltage, a low voltage rectifier circuit that converts the AC low voltage to a DC low voltage, and a control circuit that integrally controls these circuits. In many cases, these circuits are accommodated in a single housing so as to be easily mounted on the vehicle (for example, refer to PTL 1).
However, in this configuration, since the circuit that performs the conversion from DC to AC necessarily exists at the step of the conversion and the handling power is required to be reduced in a high percentage, there occurs a problem in that a noise which is generated from the DC-DC converter influences the other devices (so-called an emission noise). In addition, there is also a problem in that the DC-DC converter erroneously operates due to the influence of a noise (so called an immunity noise) received from the vehicle side or an external environment.